The title mol­ecule, C9H7NO3, exists in the diketo form and the iso­quinoline unit is approximately planar (r.m.s. deviation = 0.0158 Å). In the crystal, mol­ecules are linked into inversion dimers through pairs of O—H⋯O hydrogen bonds and are further assembled into the (100) layers via stacking inter­actions [centroid–centroid distances = 3.460 (3) and 3.635 (4) Å].

The title compound is known to inhibit metalloenzymes such as influenza endonuclease (Parkes et al., 2003), HIV-1 reverse transcriptase RNase H (Hang et al., 2004), and HIV-1 integrase (Billamboz et al., 2008). Here we report the crystal structure of the title compound, which was obtained from the deprotection of 2-benzyloxyisoquinoline-1,3(2H,4H)-dione by the use of boron tribromide. The compound exists in keto form and the isoquinoline ring is almost planar (r.m.s. deviation = 0.0158 Å). In the crystal, the molecules link through intermolecular O–H···O hydrogen bonds and stack along the c axis, as shown in Figure 2. The distance from plane1 (C7/C8/C9/C11/C12/C13) to plane2 [C4/C6/C7/C8/C10/N3, (1 - x, 2 - y, 1 - z)] is 3.460 (3) Å.

The title compound was synthesized according to the literature (Billamboz et al., 2008). Single crystals suitable for X-ray diffraction were obtained by slow evaporation of an acetone solution of the compound at room temperature.

Refinement. Refinement was performed using all reflections. The weighted R-factor (wR) and goodness of fit (S) are based on F2. R-factor (gt) are based on F. The threshold expression of F2 > 2.0 σ(F2) is used only for calculating R-factor (gt).

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